The invention concerns an optical pick-up for reading and/or writing an optical recording medium and/or magneto-optical recording medium, whereby a source projects light on the medium and light reflected by the medium is diverted to one photodetector or another depending on how it is polarized.
The invention also concerns an optical pick-up for reading and/or writing an optical recording and/or magneto-optical recording medium, whereby a source projects light on the medium.
One known example of an optical recording medium is a compact disk, which has a transparent layer on top of a reflecting aluminum layer. The light-reflecting aluminum layer has depressions called "pits" that represent data stored on the disk. The data can be read from the compact disk by means of an optical pick-up because of the reflectivity of the light-reflecting aluminum layer depends on the pattern of the depressions in the disk. Less light is reflected from a depression, which is also often called a "groove," than from an elevation, which is often called "land."
From the intensity of the light reflected by the disk accordingly, the optical pick-up determines whether the bit being scanned is a logical one or a logical zero for example.
Another optical pick-up medium of this type, called an "optomagnetic disk," is described in the article "Magnetooptische Versuche dauern an" ["optomagnetic testing continues"] on pages 37 to 41 of Funkschau 13, 21 (June 1986).
A magneto-optical disk, in contrast to a conventional compact disk, has sometimes no pits. Below the transparent layer is a magnetic layer, in which data can be stored and from which they can be read out. How data can be written onto an optico-magnetic disk will now be described.
A laser beam focused on the disk heats the magnetic layer to above the Curie point. It is, however, usually sufficient to heat the layer to a compensation temperature that is just below the Curie point. An electromagnet is positioned behind the focus on the disk and magnetizes the area heated by the laser in one polarity or another. As the temperatures of the heated points drop below the Curie point again when the laser beam is turned off, the magnetic polarity established by the electromagnet remains in force. It "freezes in." The individual bits are in this way stored in domains of differing magnetic polarity, with one polarity for example representing a logical one and the other a logical zero.
The data can be read out by exploiting the Kerr effect. The plane of polarity of a linearly polarized beam of light is rotated through a measurable angle when reflected by a magnetized domain. Depending on the direction that the domain is magnetized in, the plane of polarization of the reflected beam will be rotated either left or right. Since, however, the recorded domains on the disk act like magnetized mirrors, the plane of polarization of a scanning beam of light will be rotated right or left to a measurable extent depending on the magnetic polarity of the domains being scanned at that instant.
From the rotation of the plane of polarization of the beam of light reflected from the disk the optical pick-up determines whether the bit is a logical one or a logical zero. In contrast to a compact disk, an optico-magnetic disk can be erased and rerecorded almost as often as desired.
One innovative new type of disk is a combination compact disk and optomagnetic disk. Data can be stored on this type of recording medium in both the magnetic domains and in pits forms.